1. Field of the Invention
The present invention relates to a method for surface treatment of a skirt portion of a piston for an internal combustion engine and a piston for an internal combustion engine treated by the method. More specifically, the present invention relates to a method for surface treatment of a skirt portion of a piston for an internal combustion engine (hereinafter also simply referred to as “skirt portion”) in which initial break-in characteristics are satisfactory to achieve a reduction in frictional resistance with respect to an inner wall of a cylinder and in which wear resistance, low frictional resistance, and anti-seizing properties can be maintained even with long-term use, and also relates to a piston for an internal combustion engine treated by the method.
2. Description of the Related Art
Global warming is escalating, and a reduction in emission of CO2 gas, a greenhouse gas, is becoming a worldwide challenge today. Accordingly, it is strongly required to reduce the emission of CO2 gas also in automobiles, which are CO2 gas emission sources, and a reduction in the amount of emission gases, and therefore CO2, gas has been actively attempted by decreasing fuel consumption by reducing weight of automobiles and reducing friction of sliding parts, as well as improving a powertrain (driveline part).
Under such circumstances, reducing the weight of each part constructing an automobile and reducing the friction of sliding parts are unavoidable challenges, and, at the same time, it is required to reduce a cost of automobiles due to escalation of global competition and to exploit developing-country markets. Therefore, the reduction in weight of automobile parts and the reduction in friction of sliding parts that should be accomplished with the reduction in CO2 gas emissions must be achieved at low cost.
From these viewpoints, among automobile components, reducing the weight and friction of a piston, which is an engine part directly affecting the emission of CO2 gas, is a challenge that must be achieved. In particular, in a piston that reciprocates inside a cylinder at high speed, an increase in mechanical frictional resistance directly leads to energy loss.
In the piston of an internal combustion engine, a skirt portion in sliding contact with the inner wall of the cylinder largely affects the friction; accordingly, a reduction in the friction has also been attempted by applying a coating or the like having a low friction coefficient onto the skirt portion. As an example, it has also been proposed to try to reduce the friction of a piston by forming a coating layer made of a material whose friction coefficient with respect to a cylinder is smaller than that between the skirt portion and the cylinder, for example, a resin or molybdenum disulfide, into a predetermined shape with a die in advance and attaching the coating layer to the outer circumference of the skirt portion provided with a striation helically extending on the outer circumferential surface with an adhesive or the like (Japanese Patent KOKAI (LOPI) No. 2005-330941).
In the above-described conventional technology, friction generated by sliding contact of the skirt portion with the cylinder inner wall can be reduced by applying the coating having a low friction coefficient onto the skirt portion.
However, since the reduction in friction by the method described above is achieved by the low friction coefficient characteristics of the coating, a low friction effect cannot be obtained when the coating layer is abraded or peeled off.
Therefore, in order to maintain the friction reducing effect of the coating for a long time, it is necessary to prevent the occurrence of peeling by firmly attaching the coating to the surface of the skirt portion.
In addition, the reduction of friction by a coating as described above is achieved in many cases for enhancing the initial break-in of a piston, and in cases where the coating is formed for such a purpose, the coating is abraded by operation for a predetermined time (for example, break-in operation) and is lost. Consequently, after the loss of the coating by abrasion, the material of the skirt portion, such as an aluminum alloy, comes into direct contact with the inner wall of the cylinder. However, it is certainly required to reduce friction and prevent the occurrence of seizing also under such circumstances.
Incidentally, it is assumed that the adhesive force of a coating layer formed on the skirt portion surface is usually achieved by mechanical adhesion called an anchor effect (fastener effect) and a physicochemical bond such as a hydrogen bond or a van der Waals bond. Therefore, it is predicted that if the adhesive force based on these principles can be increased, a coating that is difficult to peel off from the skirt portion of the piston will become possible.
Here, the enhancement of the adhesive force exhibited by the anchor effect is caused by mechanical scratches generated by a film of a coating agent or by an adhesive penetrating into recesses on the surface of the skirt portion, solidifying and becoming rooted therein.
In order to increase the adhesive force due to the anchor effect, it is preferred to form asperities, on the surface of the skirt portion, into which the film or the adhesive penetrates or with which the film or the adhesive is joined, but this requires modifying the surface condition of the skirt portion. In addition, the required work and processing for forming the recesses on the surface of the skirt portion cause an increase in cost.
Furthermore, when the recesses are formed in the skirt portion for obtaining the anchor effect, the asperities on the surface of the skirt portion may also appear on the surface of a coating formed on the surface of the skirt portion. Therefore, the surface roughness of the coating surface is increased, and, as a result, friction is increased.
On the other hand, of the above-described physicochemical bonds, the hydrogen bond is a bond generated when the piston base material serving as an adherend, a coating agent, or an adhesive has active hydrogen in its molecules. For example, as shown in FIG. 10A, the hydrogen bond is generated by a hydroxide (OH) or an oxide (O) created in an Al component of the skirt portion bonding with O or H of the coating agent (for example, a polyamide-imide resin), which exhibits relatively high bonding force.
Furthermore, the bond due to van der Waals force, which is another physicochemical bond, is a bonding force created by interaction between a constituent molecule of the skirt portion and a constituent molecule of the coating layer. Even in a molecule that is electronically neutral and nonpolar, exhibiting hardly any dipole moment, electron distribution in the molecule is not steadily maintained under symmetrical and nonpolar conditions and instantaneously becomes asymmetrical. In this example, as shown in FIG. 10B, an electrical dipole (dipole moment) is formed by the positively polarized C and the negatively polarized O of a functional group of the constituent molecules of the coating layer; O and H of the hydroxyl group (OH) of an Al hydroxide constituting the skirt portion are respectively polarized negatively and positively; and O− of the functional group and H+ of the hydroxyl group bond to each other to create a bonding force.
Accordingly, in order to generate such a physicochemical bond, the surface of the skirt portion is required to be kept clean. Contamination of the surface causes a lack of adhesive force, and in order to obtain adhesive force due to the physicochemical bond, it is necessary that the skirt portion be washed and then subjected to a drying step, as preceding steps of the coating. This work is complicated and also leads to an increase in cost as a result of consumption of washing solutions, waste liquid treatment, or the like.
In addition, the aluminum alloys used in casting are mostly of a eutectic composition. Therefore, the base material microstructure of the manufactured piston is composed of Al dendrite crystals that are first produced from molten metal in the process of cooling and then grow large according to the cooling and a eutectic microstructure of Al and an alloy component such as Si, Cu, or Mg, which is generated when the remaining molten metal, whose alloy component concentration is relatively increased by the production of the dendrite crystals, reaches the eutectic composition. Therefore, segregation occurs in the distribution of the alloy components, and also coarsening of the microstructure readily occurs. In particular, these tendencies are high near a surface that is rapidly cooled by being brought into contact with, for example, a mold.
In such an aluminum alloy having segregation in the distribution of the alloy components and the coarsened microstructure, a region where physicochemical bonds with the coating is strong and a region where physicochemical bonds with the coating is poor are generated, thereby destabilizes the adhesive force. Consequently, peeling of the coating readily occurs, starting from the region of weak adhesive force.
In addition, such an aluminum alloy having segregation of the alloy components and coarsened microstructure is fragile, and the wear resistance when the base material is brought into direct contact with the cylinder inner wall due to, for example, abrasion of the coating is low.
Regarding the above-mentioned points, it is believed that in the piston described in '941 cited above the adhesive force can be increased because the surface area of the skirt portion is increased by forming a helical striation in the skirt portion.
However, when a coating layer is formed by, for example, direct application of a resin material onto the skirt portion surface on which a striation is formed, asperities corresponding to the striation appear also on the surface of the coating layer, which increases surface roughness and increases friction under fluid lubrication between the cylinder inner wall and the skirt portion surface.
Therefore, in the invention disclosed in '941, in order to prevent the striation formed in the skirt portion from appearing on the surface of the coating layer, the coating layer is molded in advance by casting a molten resin in a die (mold), and this coating layer is attached to the skirt portion with an adhesive ('941, Paragraphs [0035] and [0036]). Therefore, the work is complicated and expensive compared with the case where the coating layer is formed by, for example, directly applying a resin material onto the skirt portion.
Furthermore, in order to enhance the adhesive property of an adhesive, as described above, since the surface of the skirt portion is required to be kept clean, steps such as washing are necessary. In addition, the invention disclosed in '941 does not include a configuration that can enhance the adhesive force caused by the above-described physicochemical bonds.
Furthermore, the invention disclosed in the above-mentioned '941 describes that, even if the coating layer provided on the skirt portion is peeled off, since contact between the base material and the cylinder inner wall occurs at a peak portion of the helical striation, plane-to-plane contact is not formed, and thereby the occurrence of seizure can be prevented.
However, when the skirt portion surface having asperities due to the formation of the helical striation and the cylinder inner wall are in sliding contact with each other, the frictional resistance between the two is increased. This significantly increases the friction in sliding contact against the cylinder inner wall.
In addition, since the base material of the skirt portion is not subjected to any treatment for achieving high wear resistance or reducing friction, even if the worst condition, i.e., the occurrence of seizure, can be temporarily avoided, a large loss in energy and also wear of the skirt portion and the cylinder inner wall are caused by continuous operation in the state where the base material of the piston is in direct contact with the cylinder inner wall due to the peeling or wearing of the coating layer. Therefore, the life of piston in the above-mentioned configuration is virtually brought to an end with the peeling of the coating layer.
Therefore, this technology cannot be applied to a piston that is designed to be continously used after wearing or peeling of the coating, for example, as in the case where the skirt portion is subjected to coating for enhancing the initial break-in characteristics of an engine.
Accordingly, the present invention has been accomplished for solving the defects in the above-described conventional technologies, and it is an object of the present invention to provide a method for surface treatment of a skirt portion of a piston for an internal combustion engine, wherein a lubrication layer of a resin coating having a friction reducing effect can be made to firmly adhere to the surface of the skirt portion at relatively low cost and with a simple process, the frictional resistance against the cylinder inner wall can be reduced even if the lubrication layer is peeled or abraded, and effects of providing excellent wear resistance and anti-seizing properties and of reducing the aggressiveness against the counterpart (cylinder inner wall) in sliding contact therewith (reduction in the degree of wear of cylinder inner wall) are obtained, and to provide a piston for an internal combustion engine treated by the method.